The present invention relates to radiation curable printing inks. More specifically, the present invention relates to radiation curable printing inks of ink jettable viscosities that incorporate a radiation curable diluent and a radiation curable, polyester polyurethane oligomer having atypically low viscosity characteristics.
Inkjet imaging techniques have become very popular in commercial and consumer applications. Ink jet printers operate by ejecting ink onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink jet printers can produce a wide variety of printed features, including text, graphics, images, holograms, and the like. Moreover, ink jet printers also are capable of forming three-dimensional objects as well, which is a capability suitable in applications such as rapid prototyping.
Thermal ink jet printers and piezo inkjet printers are the two main types of ink jet systems in widespread use today. For both approaches, inks must meet stringent performance requirements in order for the inks to be appropriately jettable and for the resultant printed features to have the desired mechanical, chemical, visual, and durability characteristics. In particular, inks must have relatively low viscosity when jetted, yet must be able to form accurate, durable images on the desired receiving substrate. For example, a typical ink for thermal ink jetting must typically have a viscosity in the range of 3 to 5 centipoise at 25xc2x0 C., while piezo inks must typically have a viscosity in the range of 3 to 30 centipoise at the printhead temperature. The need to use low viscosity inks makes it challenging to obtain printed features with good mechanical, chemical, visual, and durability characteristics.
Solvent-based and water-based jettable inks are well known. A typical water-based ink generally comprises water, a colorant, which may be a dye and/or a pigment, one or more co-solvents, and one or more additives that are included to enhance the performance of the ink. Representative examples of such additives include one or more colorants, slip modifiers, thixotropic agents, foaming agents, antifoaming agents, flow or other rheology control agents, waxes, oils, plasticizers, binders, antioxidants, fungicides, bactericides, organic and/or inorganic filler particles, leveling agents, opacifiers, antistatic agents, dispersants, and the like.
Water-based inks have drawbacks. For industrial applications, drying is energy and equipment intensive. Drying water also takes time, and the printed material needs to be handled carefully during the relatively lengthy drying period. Water-based inks are also compatible only with a limited range of substrates, typically those on which the water is absorbed to some degree. Images formed using water-based inks typically require a protective overlaminate for outdoor applications.
Instead of water, other solvent-based inks include relatively volatile, organic solvents. Such inks dry more rapidly and easily than aqueous inks. However, such solvents may be toxic, flammable, or the like, requiring careful handling. These inks also tend to be compatible with only a limited range of substrates.
In order to avoid using a conventional solvent, ink compositions incorporating a radiation curable, polymerizable diluent have been developed. The diluent not only functions as a solvent, but also functions as a viscosity reducer, as a binder when cured, and as a crosslinking agent. In the uncured state, these compositions have a low viscosity and are readily jetted. However, the polymerizable monomers readily crosslink upon exposure to a suitable source of curing energy, e.g., ultraviolet light, electron beam energy, and/or the like, to form a crosslinked polymer network. Depending upon the kind of monomers incorporated into the diluent, the resultant network may provide the printed features with durability, flexibility, elasticity, gloss, hardness, chemical resistance, stiffness, combinations of these, and the like.
Reactive diluents formed from radiation polymerizable monomers have some drawbacks. First, printed features formed from these materials might have a tendency to shrink when cured. Further, the adhesion, weatherability, resilience, toughness, flexibility, dot gain, and the like also may not be as good as is desired. Accordingly, previous reactive diluents have incorporated radiation curable oligomers in attempts to improve one or more of these characteristics. The use of urethane oligomers, particularly polyester urethane oligomers would be greatly desired inasmuch as such urethane materials tend to have excellent durability, weatherability, resilience, adhesion compatibility with a wide range of substrates, flexibility, and/or the like. However, adding an oligomer to the reactive diluent tends to cause the viscosity of the reactive diluent to increase. Polyester urethane oligomers, in particular, tend to have relatively high viscosity characteristics that increase as the urethane content of the oligomer is increased. The high viscosity associated with these oligomers is a serious drawback for ink jetting applications, where low viscosity is critical to jettability. Other urethane oligomers, particularly those containing ether groups, might have low viscosity, but suffer from limited compatibility with different kinds of substrates and poor weatherability characteristics.
The present invention provides a low viscosity, radiation curable urethane oligomer, preferably a radiation curable polyester urethane oligomer, that can be incorporated into radiation curable ink compositions. Preferred embodiments are suitable for ink jetting applications. The ink jettable embodiments are particularly well-suited for use in piezo ink jet printers. The viscosity characteristics of the compositions are low enough so that conventional solvent is not required in order to satisfy the requisite low ink jet viscosity specifications.
The oligomers of the present invention also provide inks with good adhesion to a wide range of different substrates, including the commonly used, nonporous, polymeric substrates used for outdoor signing applications. Such substrates may be films based upon polyvinyl chloride, poly(meth)acrylates, polyurethanes, polyolefins, alkyd resins, combinations of these, and the like. In particular, the oligomers provide good adhesion to polymethyl methacrylate substrates, which are typically difficult to adhere to, yet are highly desirable for outdoor applications. While not wishing to be bound by theory, it is believed that the polyester components, especially polycaprolactone ester components of the oligomers, helps both to reduce viscosity of the oligomer and to promote adhesion to these substrates. The combination of low viscosity, broad range compatibility with many substrates, and weatherability is atypical of conventional polyester urethane oligomers.
Preferred embodiments of the low viscosity, radiation curable urethane oligomers are transparent and colorless, making them readily compatible with colorants that might be incorporated into ink compositions of the present invention. Preferred embodiments may also be prepared from weatherable ingredients, making the oligomers and resultant ink compositions very well suited to outdoor applications. The oligomers also enhance the flexibility of cured ink compositions, allowing the inks to conform to irregular or moving surfaces. In addition to ink jet applications, the compositions of the present invention may also be used in a wide variety of other coating applications such as screen printing, clear coats, varnishes, paints, stains, and the like.
In one aspect, the present invention relates to a radiation curable ink jettable ink composition. The ink compositions includes an oligomer that is a reaction product of ingredients comprising an aliphatic polyisocyanate comprising two or more isocyanate groups; and a radiation curable alcohol comprising one or more radiation curable moieties, one or more hydroxyl moieties, and one or more polycaprolactone ester moieties.
In another aspect, the present invention relates to a printing ink with an ink jettable viscosity. The ink comprises a reactive diluent and a radiation curable polyester urethane oligomer. The oligomer is a reaction product of ingredients comprising a polyisocyanate and a radiation curable polyester alcohol. The alcohol comprises at least one radiation curable moiety, at least one hydroxyl group, and a polyester moiety comprising at least two ester groups. Preferably, the oligomer has a viscosity to mass index below about 25:1.
In another aspect the present invention relates to a method of making a printing ink with an ink jettable viscosity. A radiation curable, polyester urethane oligomer is provided. Preferably, the oligomer has a viscosity to mass index below about 25:1. The oligomer is a reaction product of ingredients comprising a polyisocyanate and a radiation curable polyester alcohol. The alcohol comprises at least one radiation curable moiety, at least one hydroxyl group, and a polyester moiety comprising at least two ester groups. The oligomer is blended with ingredients comprising a sufficient amount of a reactive diluent such that the ink has an ink jettable viscosity.
In another aspect, the present invention relates to a printing method. A radiation curable ink is ink jetted onto a receiving substrate to form at least one printed feature. The radiation curable ink comprises a reactive diluent and a radiation curable, polyester urethane oligomer. The oligomer is a reaction product of ingredients comprising a polyisocyanate and a radiation curable polyester alcohol. Preferably, the oligomer has a viscosity to mass ratio below about 25:1. The printed feature is exposed to a curing amount of radiation.
In another aspect, the present invention relates to a method of making a printing ink with an ink jettable viscosity. A radiation curable, polyester urethane oligomer is provided that is a reaction product of ingredients comprising an aliphatic polyisocyanate and a radiation curable alcohol. The alcohol includes one or more radiation curable moieties, one or more hydroxyl moieties, and one or more polycaprolactone moieties. The oligomer is blended with a sufficient amount of a reactive diluent and other optional ingredients if desired such that the ink has an ink jettable viscosity.
In another aspect, the present invention relates to printing method. A radiation curable ink is ink jetted onto a receiving substrate to form at least one printed feature. The ink comprises a radiation curable, polyester urethane oligomer that is a reaction product of ingredients comprising an aliphatic polyisocyanate and a radiation curable alcohol. The alcohol includes one or more radiation curable moieties, one or more hydroxyl moieties, and one or more polycaprolactone moieties. A sufficient amount of a reactive diluent also is included such that the ink has an ink jettable viscosity. The printed feature is exposed to a curing amount of radiation.